In a wireless communication system, a transmitter transmits a wireless signal to a receiver through a physical channel, such as air, in the form of electromagnetic waves. Due to the unideal channel effect, such as multipath reflection and multipath fading, the wireless signal received by the receiver may get distorted.
Based on the orthogonal frequency division multiplexing (OFDM) modulation technology for the multi-carrier modulation, the effective processing ability is obtained with respect to the multipath reflection effect. In the OFDM systems, the receiver only needs one simple one-tap equalizer to equalize the frequency selective fading, which is caused by time invariant multipath channel. So, the OFDM system has recently become the mainstream technology in the communication field and the broadcasting application development. Various systems, such as the asymmetric digital subscriber line (ADSL), the power line communication (PLC), the digital audio broadcasting (DAB), the wireless local area network (WLAN) 802.11a/b/g/n, the China mobile multimedia broadcasting (CMMB), the digital video broadcasting-terrestrial (DVB-T), the digital video broadcasting-handheld (DVB-H) and the Wi-Max IEEE 802.16e with the mobility apparatus, use the OFDM transmission technology.
Nowadays, receivers of some OFDM systems (e.g., DVB-T, DVB-H, IEEE 802.16e, CMMB and the like) are emphasized to provide desirable capability of reception at the high speed motion. However, when the receiver of the OFDM system is not stationary and is moved at the high speed relatively to the transmitter, the channel in the duration of one OFDM useful symbol is no longer kept in the fixed state, thereby causing the time-selective fading channel. Due to the influence of the Doppler effect at the high speed motion, the OFDM signal is positively or negatively offset by one time of Doppler frequency (fd) with the center carrier frequency (fc) serving as the center. This offset is unfavorable to a multicarrier modulation system, and may cause the inter-carrier interference (ICI) effect that destroys the orthogonality of the sub-carriers, hence resulting in an error floor phenomenon in the performance of bit error rate (BER).
The ICI cancellation method is disclosed in US patent US 2006/0239367 A1. Combination of the windowing (e.g. 2-step/4-step windowing) and one parallel interference cancellation (PIC) equalizer is proposed of the OFDM system. In the receiver, the information of maximum channel delay is estimated to determine the information of the inter-symbol interference free (ISI-free) region and a set of 2-step/4-step windowing coefficients. According to the information of the ISI-free region, the time-domain received signal samples are multiplied by windowing coefficients, which do combination in the time domain. The combined signal is transformed into frequency domain signal by fast Fourier transformer (FFT) and then operates to cancel ICI effect by the PIC equalizer. This PIC equalizer estimates all the transmitted data according to one-tap least square (LS) zero forcing (ZF) or minimum mean square error (MMSE) filtering rule, and then performs the ICI reconstruction and cancellation operations over one unit time. The PIC equalizer has the advantages of the high operation speed, but tends to generate the estimation error with respect to the data on the sub-carrier with low signal to interference-pulse-noise ratio (SINR), thereby decreasing the accuracy of data detection.
The IEEE Transactions on Communications, Vol. 49, PP. 1375-1387, August 2001, disclose an ICI cancellation method. In this article, a successive interference cancellation (SIC) equalizer is adopted to replace the conventional equalizer, which does not consider the ICI effect. This SIC equalizer arranges the order of detecting the data on the sub-carriers according to the SINR of each sub-carrier, and then performs the data detection in order. As the SIC equalizer detects the data of one sub-carrier, the reconstruction and cancellation operations of the corresponding ICI effect are performed. Because the data detection is performed according to the order of the SINRs, the accuracy of data detection is enhanced, the accuracy of reconstructed ICI is relatively enhanced, the estimated error rate of the received signal with the lower SINR after the ICI effect is cancelled is decreased, and the performance after the ICI effect is cancelled is enhanced. However, assume the number of total sub-carriers is N, SIC equalizer needs to perform the inverse matrix operation for the size of N×N to estimate the data on the sub-carrier by using MMSE filtering rule, and performs N times of the successive interference cancellation operations according to the order of SINRs mechanism. So the overall calculation loading is relatively high, O(N4) complex multiplication operations are needed. Besides, the operation processing speed is relatively slow, hence the complexity of hardware implementation is too high.
The IEEE Transactions on Consumer Electronics, Vol. 5, PP. 2528-2532, May 2006, further disclose an improved SIC equalizer. This improved SIC equalizer directly chooses the values on the diagonal lines in the frequency domain channel matrix for the magnitude sorting, thereby decreasing the calculation complexity as compared with the SINR sorting. Meanwhile, it is observed, from the frequency domain channel matrix, that the non-zero values caused by its ICI effect in the frequency domain channel matrix distribute over the neighboring D sub-carrier data on the diagonal lines. And, the value of D is much smaller than the number of total sub-carriers (N). The frequency domain channel matrix could be regarded as a sparse matrix. When an inverse matrix operation is used to estimate data for MMSE filtering rule, the original frequency domain channel matrix with the size of N×N may be simplified into the frequency domain channel matrix with the size of (2D+1)×(4D+1) according to this property, thereby reducing the calculation complexity. Next, by using MMSE filtering rule, this improved SIC equalizer estimates the data according to the sorting result. Although the calculation complexity of this improved SIC equalizer has been reduced, the greater value of D is still needed to enhance the system performance because its ICI effect is not further centralized on the neighboring sub-carrier data. Thus, its calculation complexity is moderate. Meanwhile, N times of inter-carrier interference reconstruction and cancellation operations still have to be performed successively, so the operation processing speed is low.